1) Field of the Invention
The invention relates to a method of blowing drying gas against a paper web, in which method drying gas is blown with an impingement dryer comprising a plurality of profiling chambers in the cross-direction of a paper machine, the cross-profile of the paper web being controlled by means of the drying gas blown from the profiling chambers; each profiling chamber blowing drying gas to its own effective area; and the impingement dryer further comprising a return air chamber and return air ducts in such a way that drying gas blown against the paper web from the profiling chambers is returned into the return air chamber through the return air ducts.
Further, the invention relates to an impingement dryer of a paper machine, comprising a plurality of profiling chambers in the cross-direction of the paper machine, the profiling chambers being arranged to control the cross-profile of a paper web in such a way that each profiling chamber is arranged to blow drying gas against the paper web to its own effective area; and the impingement dryer further comprising a return air chamber and return air ducts in such a way that drying gas blown against the paper web is arranged to be returned into the return air chamber through the return air ducts.
2) Description of Related Art
A paper web produced is dried in the dryer section of a paper machine before it is reeled to the reeler of the paper machine. The dryer section typically comprises several dozens of cylinders through which the paper web travels supported by the dryer fabric. The cylinders of the dryer section are hot steam-heated cylinders that evaporate moisture from the web while the web is travelling through the cylinders. Apart from steam-heated cylinders, at least part of the cylinders are vacuum rolls, in other words what are called vac rolls. In the vacuum rolls, vacuum prevails that sucks the paper web into contact with the fabric, whereby the web moves over to the next dryer cylinder without interruptions.
A problem with the drying of a paper web is the control of the moisture profile of the web cross-profile in such a way that the moisture profile of the web stays as desired, as regards both the runnability of the web and the preservation of the web profile properties in the paper machine itself, but also during the storage, transport and end use, such as printing, of the paper. Presently, a steam box mounted on a press before the dryer section of the paper machine or a dampener positioned in the dryer section is used to correct the moisture profile of paper. A steam box or a dampener is not, however, applicable to all points of the dryer section. In addition, moistening the web is uneconomical and lowers the overall efficiency of the paper machine, because in the dryer section, the intention is to dry paper as efficiently as possible; however, adding water to the web makes it necessary to use part of the drying capacity to remove this added water.
To make the removal of moisture from the web more efficient, paper machines also utilize impingement-blowing units positioned in the dryer section. The impingement-blowing unit consists of a vacuum roll in connection of which there is an impingement dryer. As regards its diameter, the vacuum roll of the impingement dryer may be greater than a conventional drying cylinder. The nozzle surface of the impingement dryer is at a certain distance from the surface of the roll, whereby a drying zone is formed between the dryer and the roll. When the paper web travels supported by the dryer fabric through the drying zone, hot air is blown to the web from the dryer. Most of the air blown towards the paper web is returned to the dryer to be heated and re-blown towards the web. In order to maintain moisture of the drying air to be blown at a desired level, part of the moist drying air returned from the drying zone is discharged as exhaust air and replaced with a required amount of fresh substitution air. Control parameters used in the impingement-blowing drying are usually the blowing temperature and the blowing rate. In a known dryer that is arranged in connection with a steam-heated drying cylinder, the blowing chamber of the dryer is divided into profiling chambers in the cross-direction of the paper machine, whereby drying air can be blown from the profiling chambers to the paper web to the effective area of each profile chamber. This type of an impingement dryer has, however, the problem that the drying air blown from one profiling chamber is spread in the cross-direction of the paper web also to the effective area of the adjacent profiling chambers, which weakens the profiling effect of the dryer. Present impingement dryers has the problem that not even with them is it possible to achieve sufficiently accurate control of the paper web cross-profile, which would be required by the modern production and quality standards.